The present invention relates to a magnetic detector device for detecting the radial displacements of a rotor mounted on active magnetic bearings, of the type comprising a plurality of inductive elements with variable air gap, which are integral with the stator on which are mounted said bearings, and at least one reference ring which is integral with the rotor and is placed in facing relation to said inductive elements to define with them the said variable air gap.
Various known detectors of the radial displacements of a rotor mounted on magnetic bearings are used for delivering electric error signals which, through a servo-control device, enable the return of the rotor to a predetermined position. The various position detectors may be of optical, capacitive or magnetic type.
One example of magnetic detector and of detection device, combining a plurality of magnetic detectors constituted by inductive elements working in cooperation with a reference ring integral with the rotor, is described in French Patent No. 2 214 890. The detection device described in said patent is solely concerned with detecting the variations of so-called reference distances which are due to a true radial displacement of the rotor axis, and with reducing the detection of the distance variations due to a variation of the radius of the reference ring of the detection device.
It is indeed extremely difficult to produce cylindrical reference rings of rigorous circularity. Now , when a detector device detects distance variations which are due to a variation of the radius of the reference ring coupled to the rotor, and do not represent a true radial displacement of the rotor, damaging vibrations may be induced in the magnetic suspension and the consumption of energy of the device receiving the output of the detector device to servocontrol the active magnetic bearing is increased. It is therefore important to have an efficient detector device with a good rate of elimination of the harmonics of the Fourier's series decomposition of the curve representing, as a function of the corresponding polar angle, the true form of the outline of the reference ring of the rotor which faces the detector device.
Although the device described in French Pat. No. 2 214 890 is a definite improvement over previously known devices, it does not, however, succeed in reducing the influence of the odd harmonics beyond the third harmonic and it requires the use of a large number of basic inductive elements with radial flux constituted by small pieces having an axial plane of symmetry and being situated inside a radial plane. These different elementary pieces may be grouped together and may be constituted by a disk composed of stacked plates and having notches in which a conductor wire is wound. However, such an embodiment is hardly suitable for detection devices designed to equip small machines requiring a detector of small diameter. The provision of small notches indeed contributes to make production very difficult, and the rate of elimination of higher harmonics remains low.